1. Field of the Disclosure
The present disclosure relates to a seal for use in machines and more specifically, although not exclusively, to a seal for sealing between opposing walls of an internal cavity of a machine.
2. Description of the Related Art
FIGS. 1 and 2 show respectively a longitudinal section and three-dimensional view of a conventional gas turbine engine 10. The engine 10 comprises a core engine 12 and a nacelle 14 surrounding the core engine 12 and a propulsive fan 16. The space between the core engine 12 and the nacelle 14 defines a generally annular bypass duct 18 such that in use part of the airflow from the fan 16 enters the core engine 12 and the remainder passes along the bypass duct. A majority of the airflow along the bypass duct 18 flows directly aft to provide propulsive thrust.
The gas turbine engine 10 comprises a plurality of ventilation volumes, typically referred to as zones, created in the space between the engine, nacelle and bypass duct. The flow to/from those volumes is controlled by arrangements of inlets(s) and outlet(s) into the respective zones, or portions thereof. In this manner the flow rates from the bypass to the core of the engine for cooling and ventilation purposes are controlled. Three such zones are shown and labelled 20, 22 and 24 in FIGS. 1 and 2, in order to indicate zones conventionally referred to as zones 1, 2 and 3 of gas turbine engine 10. The inlets and outlets for each zone are labelled in FIG. 1 with the suffixes ‘a’ and ‘b’ respectively and arrows shows the general direction of flow through the zones in use. Flow from zone 3 may also pass into the turbine casing cooling manifold 26.
FIG. 2 shows the extent/length of the zones 20, 22 and 24 in the direction of the primary engine axis in use, i.e. about which the fan 16 rotates.
A sealing membrane is used between these controlled/enclosed air flow volumes (zones) or between these volumes and the bypass air flow to ensure control of ventilation flow and minimised uncontrolled leakage. The choice of seal is dependent on the height and/or possible variation in the gap to be sealed. That is to say, it has been found that the combined tolerances of the components forming an assembly to one, or either, side of the seal may vary from engine to engine, thereby causing a greater or small gap across which the seal must extend. This problem typically occurs for a radial gap, e.g. due to stacked tolerances on a radially inner side of the seal, but could equally apply for an axially extending gap in other configurations.
An example of one such seal 28 is shown in FIG. 3, in which an undeformed seal profile 28a is shown in FIG. 3a. FIGS. 3b and 3c show the varying degrees of compression that must be accommodated by the seal 28 in a normal or maximal gap height in FIG. 3b or a minimal gap height (maximal compression state) in FIG. 3c. 
However the compressibility of the seal to accommodate different gap heights is only one engineering consideration. The design of a suitable seal is complicated by the need to also be able to prevent a fire destroying or otherwise crossing the seal. This safety requirement is particularly important in gas turbine engines to prevent flames propagating from the engine core 12 casing or nacelle 14 interior into the bypass 18.
In order to reduce the susceptibility of the seal to fire damage a flame shield 30 may be provided adjacent the seal 28 as shown in FIGS. 3b and 3c. However the flame shield 30 of FIGS. 3b and 3c is designed for a minimal gap height (i.e. a maximum seal compression condition) since an optimum design at minimum compression condition may result in non-acceptable wear or contact loads at maximum compression. The limitation of the extent of the flame shield 30 thus results in it being of varying effectiveness between different engines of the same type.
This compromise therefore exposes seals 28 in low or normal compression states to an increased risk of possible fire damage. However the gap height is unknown prior to assembly and so there is no satisfactory way of guarding against this risk using conventional seal arrangements other than to increase the size and/or stiffness of seal membranes beyond that which would be optimal for normal operation. This problem is compounded by the possibility of relative deflections in use and the degree of seal compression being critical to fire resistance capability.